1. Field of the Invention
This invention relates to a tuning circuit using a current-controlled variable inductor comprising a tuning coil and a control coil for controlling the inductance of the tuning coil by controlling the magnetic flux density thereof, and it also pertains to a receiver using such a tuning circuit.
With a tuning circuit for a receiver, it has heretofore been the practice to employ either variable capacitance element (variable capacitance diode) or variable inductance element to select receiving frequency. A .mu.-tuning circuit using variable inductance element is extensively employed with a receiver to be mounted in an automobile, and is predominantly arranged such that the receiving frequency is selected by adjusting the inductance of the variable inductance element by moving the magnetic core of the radio frequency coil incorporated in the variable inductance element. More specifically, such a .mu.-tuning circuit is so designed as to be tuned to the desired receiving frequency by moving the rod-like magnetic core relative to the tuning coil to control the magnetic flux density thereof, thereby changing the inductance of the tuning coil.
2. Description of the Prior Art:
To have a better understanding of the present invention, description will first be made of the front-end section of a receiver using a conventional .mu.-tuning circuit which is schematically illustrated in FIG. 1 of the accompanying drawings and which includes an antenna 1, an antenna tuning circuit 30, an RF amplifier 31, and a radio frequency tuning circuit 32. A capacitor C.sub.0 is provided to achieve impedance matching between the antenna 1 and the tuning circuit. The .mu.-tuning circuit of the antenna tuning circuit is arranged such that tuning is effected by displacing the magnetic core of the tuning coil to control the inductance of the tuning coil, and it is also designed such that the range over which the inductance of the tuning coil L.sub.10 is variable, is set up to meet such a condition as defined by the following relationship between the receiving frequency and the inductance: EQU fmax.sup.2 /fmin.sup.2 =Lmax/Lmin (1)
where fmax and fmin represent the maximum and minimum receiving frequencies respectively; and Lmax and Lmin denote the maximum and minimum values for the inductance of the variable tuning coils L.sub.10 and L.sub.11 respectively.
As mentioned above, the conventional variable inductance type tuning coil, or .mu.-tuning circuit is so arranged as to be tuned to the desired receiving frequency by manually changing the position of the magnetic core to control the magnetic flux density, or inductance of the tuning coil. Thus, the conventional .mu.-tuning circuit disadvantageous in that expensive mechanical means is required for mechanically moving the magnetic core to permit the tuning circuit to be tuned to a designed receiving frequency, instead of by using an electric signal.